Comparative morphology of leaf epidermis of Salix (Salicaceae) with special emphasis on sections Lindleyanae and Retusae

The sections Lindleyanae C.K. Schneid. and Retusae A. Kern. are two unique taxa of Salix. Although the former is distributed in the Himalayas and in south-west China's Hengduan Mountains, and the latter in the Arctic-Alpine region, they share similar phenotypic characters. To elucidate their relationships, some representative species were selected and their leaf epidermal characters were examined. Most Salix species had broadly similar leaf epidermal cell characteristics, but Lindleyanae and Retusae shared an anomocytic type of stomatal apparatus, which differed greatly from other taxa in Salix. It is unclear whether this epidermal similarity is indicative of similar environments or a single phylogenetic origin. Within the family Salicaceae, cyclocytic stomata were present in Chosenia, but absent from Populus and Salix, partly supporting the separation of Chosenia from Salix as an independent genus. Within single Salix species, obscure interspecific phenotypes could be identified by the shape of the epidermal cells, the presence or absence of stomata on the adaxial or abaxial sides, and the shape of the anticlinal walls. Stomatal size and density in Salix varied in response to environmental factors, and was of little diagnostic value in the separation of the species. (C) 2008 The Linnean Society of London.